In recent years, continuously variable transmissions (e.g. metallic belt type and troidal type) have been developed as transmissions for automobiles and the like and have been put into practical use. At first, a lubricant oil for automatic transmission was used for a lubricant oil of the continuously variable transmissions. However, more superior performance has come to be demanded for a lubricant oil in accordance with improvement in the performance of the continuously variable transmissions. Especially, since the friction characteristics of the lubricant oil used for a wet clutch of automatic transmissions are optimized for automatic transmissions, when the lubricant oil for automatic transmissions is used for continuously variable transmissions, friction coefficient between metals is likely to become insufficient to make it difficult to transmit a large capacity torque.
Therefore, various lubricant oils have been developed for continuously variable transmissions. For instance, a lubricating oil composition disclosed in Patent Literature 1 contains an alkaline earth metal sulfonate or phenate, imide compound and phosphoric compound. A lubricating oil composition disclosed in Patent Literature 2 contains: at least one phosphorous-containing compound selected from a phosphoric monoester, phosphoric diester and phosphorous monoester, the phosphorous-containing compound including a hydrocarbon group having 1 to 8 carbon atoms; and a tertiary amine compound substituted by a hydrocarbon group having 6 to 10 carbon atoms. The lubricating oil compositions disclosed in the Patent Literatures exhibit a high friction coefficient between metals for a lubricant oil for a continuously variable transmissions.